The present invention generally relates to metal-semiconductor-metal photodiodes, and more particularly to a metal-semiconductor-metal photodiode in which a dark current is minimized.
Generally, a metal-semiconductor-metal (MSM) photodiode is suited for forming a semiconductor device by integration with transistors. The present inventor has previously proposed in Japanese Patent Application No. 60-111376 an MSM photodiode in which dark current is reduced. According to the previously proposed MSM photodiode, tungsten silicide (WSi.sub.x) is used as the material for the electrodes of the MSM photodiode so as to reduce the dark current and to obtain a signal reception system which can receive signals with a satisfactory signal-to-noise ratio.
The previously proposed MSM photodiode is advantageous in that dark current is reduced and the signal reception system can receive signals with a satisfactory signal-to-noise ratio. But in the previously proposed MSM photodiode, the same electrode material is used for both anode and cathode electrodes of the MSM photodiode, and compromise is necessary when selecting the electrode material. In other words, when the Schottky barrier height from the conduction band is high, the injection electrodes are in small numbers but the injection holes are in large numbers. On the other hand, when the Schottky barrier height from the conduction band is low, the injection holes are in small numbers but the injection electrons are in large numbers. Hence, WSi.sub.x is used as the electrode material for both the anode and the cathode electrodes of the MSM photodiode. However, it would be more advantageous if it were possible to further reduce the dark current.